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If the Concorde had an engine fail at cruise altitude, would they enter the coffin corner?
I know the coffin corner is only meant for subsonic aircraft, but if the Concorde is just cruising at Mach 1 and had an engine failure and also lost that afterburner, would it enter a coffin corner situation?

I don't think thrust influence coffin corner definition. Thus engine out situation would not change anything.

@ManuH how could it not if the engine failure caused the flight to go subsonic

Your understanding of coffin corner is incorrect. See https://aviation.stackexchange.com/questions/70298/what-are-the-speeds-for-absolute-ceiling-and-coffin-corner. You are thinking of excess thrust.

Could the afterburner possibly continue to work if the main engine that it was attached to failed?

The Concorde doesn't need reheat to cruise supersonically, just to get there (the range would be pretty short if it did).  If it loses an engine it's not going to put the other 3 into reheat to hold altitude and speed (fuel burn) and it will do what just about all airliners have to do when at they lose an engine, descend to whatever its engine-out service ceiling is.  This is done at some speed near minimum drag that gives the flattest descent, called "driftdown" speed, whatever it is, and it will just pitch down as required to maintain that speed with the available military rating (non-afterburner) thrust. Obviously, it will come out of mach as it does so.
Also I should point out that being a supersonic aircraft, it doesn't really have a "coffin corner" in the normal sense.  That applies to subsonic airplanes that are limited to a speed just shy of Mach, and when they get so high they become hemmed in by stall speed on one side and trans-sonic related issues on the other.  The upper limit of a supersonic airplane's speed is aerodynamic heating related and is a function of the airframe material's ability to run at high temperature, which limits aluminum structures to below Mach 3.  This is why the SR71 is titanium.

IIRC, Concorde couldn't stay supersonic on 3 engines, and the reduction in speed (and altitude) caused a dramatic reduction in range.

Thanks for pointing that out.  I should have mentioned that driftdown speeds will be subsonic. I made some edits.

@RobinBennett The cruise thrust at Mach 2 was about 10,000 pounds per engine. The engines for production Concordes were rated at 32,000 lb without reheat. So it seems unlikely that cruise thrust couldn't be maintained on 3 engines at 13,300 lb. This is one of the (few) advantages of four engines compared with two, of course.

This is a matter of definition, but the notion of 'coffin corner' can easily be applied to supersonic flight. The nature of limitations doesn't matter much: if you can't fly faster nor slower, you are in the corner. (That said, Concorde's limits in the top-right corner of the envelope (M2, FL600) were mostly certification-related, and it wasn't in the 'corner' at all there in the physical sense).

@Zeus I reckon the difference is that a subsonic plane going too fast risks being instantaneously ripped apart by aerodynamic forces, wheres a supersonic one rather overheats, which is a bit slower and thus allows more time for correction.

@leftaroundabout Subsonic planes such as the 747 have gone supersonic, in a dive, and weren't ripped apart by that (definitely not instantly and not apart, though there was local damage). At their maximum speed, they already have multiple areas of supersonic flow.

@alephzero: Per http://www.concordesst.com/powerplant.html it seems that the 10k lbs is the maximum thrust they could produce in supersonic cruise. We don't know how much thrust they needed to actually maintain cruise speed and altitude, though.

Comment only (and very informally put): A U2 (more different than a Concord(e) would be hard to get :-) ) at altitude cruises in a close to zero range speed window. Stopping it flying and getting it down to land has been said to be 'the hard part'.

The coffin corner is the altitude where your maximum speed (limited by high speed buffetting) is equal to your minimum speed (limited by low speed buffetting / onset to stall).
The Concorde, when cruising at Mach 1, is not in the coffin corner, regardless of its engines health. Both the upper speed boundary and the lower speed boundary* of the flight envelope are not affected by the engine health.
*when taking non-linearity due to high angles of attack into account, the engine thrust has an effect on the minimum speed bound of the envelope. That is not relevant to the coffin corner situation, because of limited angle of attack.

If it loses an engine, I’d imagine keeping that speed would be very difficult. If the flight goes from supersonic to subsonic

Keeping speed is easy if you allow the aircraft to descent. Anyway it has no effect on the coffin corner, that stays where it was before.

Coffin corner is about speed: the small margin between stall and critical mach.
You don't need engines to sustain speed.  You can use descent instead of engines.
As you descend, the margin widens.  Soon you find an altitude where your remaining engines can power you at a viable speed.

It's also all about the airframe, and essentially irrespective of thrust. A U-2 at 80,000' with 4 knots between high & low-speed limits, isn't helped by having more & more power available; more thrust to climb much higher & the lines cross & he can be too slow (IAS) + too fast (IMN) at once! The OP's suggestion that engine loss might put one into a coffin corner reflects a complete misunderstanding of what coffin corner is, and isn't.


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Why the speedometer in Mig21 have 2 needles?
How pilots are  able to read of the speed ? It's hard enough for one needle.

I don't think it did, can you please edit and post a picture of the gauge in question?

Agree with GgD. Did a Google search for "mig-21 speedometer" and went to images...  those that are labeled specifically mig-21 have one needle...

I concur, "скорость" -gauges have only one needle. As with many fighters, AoA is of more importance than exact speed.

George Geo, your question is in the process of being closed as needing details and/or clarity. It seems you may have seen a meter other than a speedometer or perhaps from a different plane or one with a non-standard configuration. Please link to whatever your source is, or attach a picture of what you saw. Once you've shown us what you were talking about, we can help you better identify it, and reopen the question.

The Q is bad, ,but still good enough to attract a pretty good answer.

@fraxinus Indeed, evidently the question was clear enough. Voting to keep open.

The normal airspeed indicator (showing indicated airspeed, IAS) only has one needle:

The IAS indicator (US-1600) shows Indicated Air speed up to
1600km/h. It has one needle (2) and one window (1). This window
will show “1” when IAS is >1000km/h.

I think you mean the Mach meter, which is combined with a TAS (true airspeed) indicator:

Thick needle 1 shows the M number starting from 0,6M. Thin
needle 2 shows the TAS starting from 600km/h (on 0,6 mark).
Scale indication follows this logic.

Source: Flight Manual for the DCS (Digital Combat Simulator) MiG-21 by Leatherneck Simulations

I looked at the answers and by mistake I put the Mig21 21 in the title. I was Mig23 about.

@GeorgeGeo Sorry, but you cannot fundamentally change the question after it has received an answer. Please ask a new question and please include an image of what you are talking about so we don't have to guess.

source: Stack Exchange

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Has an F-16 ever done a landing without the speedbrakes extended?
Has there ever been a landing made by an F-16 without the speedbrakes extended?

Ever? In the 4+ decades that it's been flying, in dozens of different countries... it'd be more surprising if it hasn't happened. Which doesn't mean that there is a publicly accessible report. But when you have thousands of jets flying for that long, just about anything that can happen, has happened, at least once.

It's safe to assume that yes - if there was any malfunction which prevents the extension of the speedbrake. Why?

@RalphJ especially with an aircraft that even its first flight was accidental ;)

Yes, they do.  I’ve seen plenty of Viper landings where the pilot did them without using speedbrakes on final.  They will generally extend them after touchdown.

Here's the documentation from Lockheed on the F-16. From just a general glace, it looks like the only landing distance performance tables that are provided are for short-field operations (similar to how Cessna produces their POH), which all indicate that speedbrakes are extended.
Additionally, the same document lists "Speedbrakes - Close" in the after landing checklist, where it lists "Speedbrakes - Closed" in the before takeoff checklist. Deduce what you will from that, but to me this means the speedbrakes would have to be open for landing necessitating the 'close' action, not a verification that they are closed, as in the before takeoff checklist.
(Just as a side note, a podcast that I listen to quite regularly interviewed an F-16 pilot (John "Rain" Waters,) and he mentions in the podcast that the airplane was challenging to land because of its inherent desire to fly. So it would make sense then that in order to attain Vref or similar, speedbrakes would need to be extended. And it seems that way from the documentation as well.)
So, to directly answer your question: under normal procedures, no probably not.

That's the standard operating procedure, but the question asked if an F-16 has ever landed without speedbrakes. This answer doesn't address that. See the comment from Ralph J above.

@CatchAsCatchCan I did see that prior to giving my answer, which is why I took a different approach. However given the brevity of the initial question, I'm not sure you can read much into it. I interpreted the question to ask if landings are ever made during normal procedure without speedbrakes. I'm sure one would be intelligent enough to deduce that the system could fail, resulting in a landing without the system. Perhaps that's an assumption I made incorrectly. Regardless, "under normal procedures, no probably not." is asserted in my answer, thus I do address the question.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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When flying on a glide slope, does the G meter show 1G or less?
If your angle of attack in landing pattern is 14 degrees but your vertical velocity is 600 feet/minute, does the HUD show you 1G or not? Why?

I presume, an implicit assumption in your question is that we are flying a perfect glideslope, that is, a straight line with a constant speed, and without disturbances.
If this is the case, the acceleration will be zero by definition, and the only effect we (and an accelerometer) will feel is gravity.
The 'G-meter' shows the acceleration (incl. gravity) with respect to a calibrated standard 1G. Now, the question is, will gravity be the same from the point of view of the device/pilot?
The answer is: almost, but not quite.
First of all, the 'G-meter' typically shows the normal acceleration: that is, along the body Z axis. It is 'vertical' for the pilot, but not necessarily vertical for the Earth. This is the critical acceleration component, the greatest one (because the wing can produce by far the greatest force on an airplane), the one that is always specified in the flight manual and the one that you might exceed in normal manoeuvring. This is why the device shows it and not the 'total' combined acceleration.
Thus, the gravity component as measured by the 'normal' accelerometer will depend on the airplane attitude. In terms of G, the factor will be $cos(pitch) \cdot cos(roll)$. For the given example, if we assume the glideslope angle -3°, pitch will be $14-3 = 11°$, and with zero roll, the measured G will be $cos(11°) \approx 0.98$. Very small difference indeed.
However, in a more extreme example of a vertical dive, the accelerometer will show zero (0G).
But that's not all. If we want to be really pedantic and measure beyond 1%, we'll need to take into account that the apparent gravity changes depending on the location. Given that the accelerometer shows, essentially, weight of a calibrated mass, it will indicate different G even if the aircraft is standing on the ground.
I'm not talking about gravity anomalies, which are truly miniscule. The biggest contributor is Earth rotation, which adds some centripetal acceleration, the more of it the closer to the equator you get. At the equator, it reduces your weight by almost $0.4$% with respect to the pole.
A further $0.1$% can be accounted by the fact that Earth radius is greater at the equator than at the poles - by about 21 km. This is typically a bigger contributor than the aircraft's own altitude (esp. given that glideslopes are usually near the ground).
Altogether, if we calibrated at a pole and then make an approach near the equator in the stated conditions, we can expect almost $2.5$% reduction of the indicated G-force.
In truth, the calibration is usually done for middle latitudes (and digital AHRS may calibrate before takeoff on the actual location), so the difference will be even smaller. But anyway, the biggest contributor - attitude - is unaffected by these small changes. Yet, it is still small by itself, often below the accuracy of the G-meter.

" Thus, the gravity component as measured by the 'normal' accelerometer will depend on the airplane attitude. In terms of G, the factor will be cos(pitch)⋅cos(roll). For the given example, if we assume the glideslope angle -3°, pitch will be 14−3=11°, and with zero roll, the measured G will be cos(11°)≈0.98. Very small difference indeed."I believe this   pitch will be 14+3=17° real AOA into the wind

@GeorgeGeo Landing pitch is 'up', glideslope pitch is 'down' so 14-3 is correct. Either way, 17° pitch also only makes for a factor of 0.95 so hardly significant.

You said that the above horizontal line is 11 .Now I understand that. But my view is the total 17 degrees AOA  because you sink .

@George, but you specified that AoA is 14°. AoA, by definition, is with respect to air, not ground. Glideslope always goes down (from the aircraft), so we are descending, thus air comes a bit from below. For a given AoA, the pitch attitude will be lower (but still nose up).

Corect , touching down with the main landing gear first.

the G meter measures acceleration. If you are descending at constant vertical speed or climbing at constant vertical speed, it will read 1G.
The difference in the force of gravity between that at the earth's surface and that at 35,000 feet is far too small to be detected by an aircraft's G meter.

This is correct. It’s important to mention that a G meter measures accelerative force relative to the gravitic constant of earth, not actual movement. Sitting on the ground, entirely immobile, you would measure 1G. Accelerating toward the earth at 9.8 meters per second per second (m/s/s) would display 0G, or ‘relatively’ weightless. Accelerating away from earth at 9.8 m/s/s, represents a doubling of the standard accelerative force or 2G even though you are only accelerating upward at the same speed that gravity would cause you to fall.

@Aaron, the problem is, the 'G-meter' typically displays only the normal acceleration (or rather, force with respect to gravity, as you rightly say). Thus the indication will depend on attitude.

Ok. Yes. Maybe it's splitting hairs, but @Zeus is correct. It only measures the intended force perpendicular to the plane in which it is oriented - pragmatically speaking, perpendicular to the seat cushion under your butt.

But let's do an 4  point aileron roll in a level flight. Banking to the left 90 degrees the a/c will show you 0.5 G's then upside down - 1 G

Climbing at 45 degrees will show 0.6

@George, banking 90° (and stopping there for a bit) will show 0G. Inverted [level] flight is indeed about -1G. Climbing 45° (normally), assuming an additional AoA ~8°, will show cos(53°) ~ 0.6 indeed.

If you fly straight ahead and at constant speed (vertical and horiz) then the only acceleration on the airframe will be the gravity, so total 1G as you normally understand it.
Wether the instrument will actually display 1G depends on calibration (funny trivia, gravity acc. at 35'000 feet is less than 1G since you are further from the Earth) and also depends if instrument shows total acceleration or longitudinal/vertical components.
To put this another way, just because you are descending it will not show <1G, just as it will not show >1G if you are climbing. If you think about it, it would violate a lot of Einstein's relativity postulates as you would be basically be able to tell how you are moving in relation to Earth by using a simple self contained accelerometer.


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What is the nature of feathering in an ATR type aircraft?
If the engines fail for some reason midflight, does the ATR aircraft automatically feather the affected engine? In rare cases if both engines fail, does the automatic feathering happen for both propellers? If automatic feathering happens (if there’s an automatic behaviour), is the pilot notified in any way?

You must've watched the Mayday episode of the ATR that ditched in the Med after both engines quit from the wrong fuel quantity computer?

Depends when the power loss happens and the status of the opposite engine. Airplanes like ATRs and Dash 8s have Autofeather Systems, but they are only active during takeoff and climb and are inactive during cruise.
When the system is armed for takeoff, Autofeather happens when one engine's torque is below some threshold, say 20%-25%, and the other engine is above some threshold, say 50%. If the torque is lost, a solenoid valve in the propeller control system opens to dump fine pitch oil pressure in the prop of the low torque engine and force it to the feather position.
Once the system is disarmed during the climbout, an engine failure will not trigger Autofeather and the crew has to select START/FEATHER on the propeller control levers to feather the props. If that doesn't work, there is normally a backup "Alternate Feather" system that powers up a dedicated electric oil pump in each engine (powered off the battery) to drive the prop to feather even if there is no oil pressure remaining.
Autofeather systems apply another risk level, that of feathering spontaneously when you don't want them to, to the airplane, so they are only armed for the most critical performance case, takeoff and initial climb. The rest of the time, you want that possibility excluded because an engine failure without immediate feathering is not critical to flight, and the crew following its normal procedures to secure the failed engine will feather the engine the propeller then.
If both engines fail at the same time, Autofeather, even if armed, won't activate because you need the power differential between the two engines, one high and one low, for it to trigger when armed. In any case, the double engine failure case is not accounted for in the design, the probability of simultaneous internal mechanical failures being too low to have to be accounted for in the mathematically derived risk analysis that drives system architecture and design (actual double engine failure cases are almost always due to an external cause, like fuel exhaustion or some weather phenomenon).

Short answer: If the ATPCS is armed, it can auto-feather upto one engine. Both engines will never be auto-feathered.
(Disclaimer: This answer is based on the ATR 72 FCOM. That being said, the systems on ATR 42 would work in a similar way.)

ATPCS
In case of engine failure, the Automatic Takeoff Power Control System feathers the propeller of failed engine and increases power on the remaining engine; doing so is crucial in order to continue the takeoff.
However, this system only works during the takeoff and initial climb. The following are the arming conditions for ATPCS:

Power management selector at TO detent
ATPCS push button - ON
Both power levels above 49°
Both torques above 46%.

Provided that all arming conditions are met, the arming of ATPCS will be denoted by an $\small \color{green} {ARM}$ light on the engine control panel.

Engine failure during takeoff
The ATPCS provides facility to perform takeoffs with a derated power setting of 90% (100% RPM and 90% torque). If an engine fails with ATPCS armed, the following changes will take place:
On ground: An uptrim will be immediately applied on the remaining engine, which increases it's power by 10% (upto a maximum of 100%). Following a delay of 2.15 seconds, the auto-feather sequence will initiate, which includes the following items:

A valve lift solenoid will be energized, which enables the flow of feathering oil

The electric feathering pump will be energized

The failed engine will be feathered

Auto-feather will be inhibited on the remaining engine

The ATPCS $\small \color{green} {ARM}$ light extinguishes, indicating the completion of ATPCS sequence.

In flight: The feathering sequence remains same; the only difference is that 10% uptrim will not be applied in flight.
Reason for 2.15 second delay: This is in case the takeoff is rejected - the windmilling propeller will assist in slowing down the aircraft. The ATPCS does not know if the aircraft has/hasn't exceeded $V_1$, so it will always apply this delay - even in flight. If the power levers are retarded during this time period, the ATPCS will automatically disarm.

Engine failure after takeoff
During the initial climb-out, the ATPCS remains armed, and so the failed engine will feather normally. The uptrim will not be applied however, and so the pilot must manually move the power levers to the ramp.
However, soon after takeoff (at the acceleration altitude), the power management selector will be set to CLB detent, which disarms the ATPCS. Beyond this point, should an engine failure occur at any point in flight, the auto-feather will not occur, and the propeller of the failed engine will be windmilling.
In fact, the governer of the failed engine fine-out the propeller in an attempt to maintain the selected RPM, which will further increase the drag. In this case, the pilot must manually move the appropriate condition lever to FTR - to feather the propeller, and then to FUEL S.O. - to stop fuel supply to the failed engine.

Duel engine failure
With ATPCS armed: There is a possibility that one engine will auto-feather - that is, if both engines do not fail simultaneously. Provided that the torque of one engine drops below 18% while that of the other is still above 46%, the ATPCS will feather the propeller with lower torque. And again, this is possible only during takeoff and initial climb - when the ATPCS is armed.
If both engines were to fail simultaneously, none will auto-feather, since the ATPCS would automatically disarm as soon as both torques drop below 46%. The pilot must manually feather the props by moving condition levers to FTR detent.
With ATPCS disarmed: Now the auto-feather is completely disabled - none of the engines can auto-feather. Should one or both engines fail, the pilot must manually feather them.

is the pilot notified in any way?

The notification that feathering is complete is that the RPM of affected engine will rapidly drop down to zero. This is also indicated by an "auto-feather" text below the RPM indicator on the Engine Warning Display (EWD). A similar notification will be given for uptrim, displayed above the torque indicator.

 Note: Please refer to ATR 72 FCOM for more details on ATPCS and other ATR systems.


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Is there simpler way than scraping flightradar to determine Number of Flights between Two Countries?
Is there a simple way to procedurally determine how many flights are flown between two countries on a given day?  Maybe even get an inbound/outbound break down?
I consulted the FAA data site (https://www.faa.gov/data_research/aviation_data_statistics/) but I didn't find anything promising.  I was also considering scraping flight radar (https://www.flightradar24.com/) but is a somewhat complicated endeavor.  Is there a simpler solution?

resources location is off topic as defined by the help center. You'd better ask "how many flight cross a border between country X and country Y?". The answers are likely to give you a further readings and links to resources you want.

Welcome to aviation.SE! One easy but possibly not cheap option is to buy the data from a specialized research company. High quality data is valuable and it takes effort to gather and maintain it.

You could try sites like Kayak.com and see what's being offered for sale between countries.  Not sure if all airlines would be shown there, but it's a start.  Could be just as tedious as other means as the search is airport to airport, but some countries don't have that many international airports to select from for larger commercial flights.  And some do have a lot.

ADS-B Exchange could be an alternative to FlightRadar24. They have (free if you host a feeder station) APIs so you wouldn't need to go through the headache of scraping. The data should be comparable and with ADS-B becoming mandatory in more and more regions (US since the beginning of the year and an EU mandate covering pretty much all business flights coming in June) you should be able to track anything but small VFR only aircraft.

Oh this looks very promising! I think I'm going to start by querying airline websites but this seems like a better long term solution.

Not really, and for many countries there's really no way at all.
For example within the EU you could fly your Cessna between Germany and say Denmark and never get recorded anywhere that's publicly available.
Nothing on sites like Flightradar24 (at least not reliably), no official records except for your payment of landing fees using your credit card (then again, if you pay cash...).
I'm sure the same is true between some other groups of countries. And then there are countries where the border and airspace are so porous it's quite possible to get in and out without being noticed illegally. Think smuggling fligths somewhere in Africa or South America. No radar coverage, bush strips that officially don't even exist.
And those flights can make it into the USA as well (drugs runners from Colombia sometimes use aircraft and try to sneak through the US border patrol, navy, and coast guard radar pickets, some no doubt make it.


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Why don't ailerons affect pitch?
Why don't ailerons affect pitch, or do they?
I'm told spoilers can affect pitch if not placed in the right spot.
On a classic standard wing configuration, why don't ailerons affect pitch as they are at the trailing edge
and would create a moment around the spar which I'm told it usually located at 25% chord
in ultralights.

Are you interested in a specific wing configuration (you can imagine that elevon on delta wings does not react the same way as aileron with droop function such as the one used on the A320)?

Two reasons:

Because they don't have a very long moment arm.
Because their movement works in opposite directions to each other, any pitching moment one side might cause is cancelled out by a pitching moment going the other way from the aileron on the other wing.

The Kolb Ultrastar ultralight suffered a little bit from the single strut wing, who's spar was a 5" tube, twisting away from the aileron movement, which had the effect of negating part of the roll input.  Left stick twisted the right wing LE down and the left wing LE up.

So that's the ailerons having a local pitching moment on each wing, but they would (at least partially) cancel each other out and not affect the net pitching moment on the whole aircraft. This sounds like it would be bad for the lifespan of the spar though - do you know if it lead to metal fatigue?

No I don't believe so; it was just considered a quirk of the design and was only apparent with very large control inputs.  The torsional stress on a 5" .052" wall spar extrusion was insignificant.  It was just a case of the wing being a little bit springy in the torsional axis.

This isn't specific to ultralights, but more generic to airplanes.
They could, but ailerons don't have the torque to change the direction of the aircraft like the elevators would. The center of pitch change would be a line that goes through the aircraft similar to the position of the wings. The elevators being further away from the center of the aircraft can push more. Designers purposely only have the elevators move for this reason.
The ailerons move in opposite directions when the move to induce roll. The center of roll would be roughly the center of the fuselage from head to tail. The ailerons are positioned away from this point to increase their torque.
I hope this helps.

When de Havilland developed the first Cirrus-engined Moth in the mid 1920s, they included differential movement of the ailerons. The common problem they faced was was that when turning and inadvertently approaching a stall, moving the inner aileron down to pull out of the turn would actually encourage tip stall at that moment. So the aileron being lowered moved by only a small amount, while that being raised moved more authoritatively. Overall this did create a small pitch-up moment but it was minimal and did not adversely affect handling. de Havilland used it extensively afterwards, I don't know about other designers.


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Can control surfaces for slow aircraft be 3D printed?
Can control surfaces for slow aircraft be 3D printed or even just printed as a core and then wrapped in fibreglass or carbon fibre?
I understand that 3D printing of wings, etc. is pretty common in the RC Aircraft world.
What about for slow flying aircraft such as Ultralights, etc.
In Rutan's book, "Moldless Composite Sandwich Aircraft Construction", Rutan strictly notes that only certain foams can be used as a core for structural components. Other foams are just for shaping of non-structural components such as interior panels, etc.
I assume there are certain engineering/material properties of the structural components that need to be met such as flexibility, no delamination, no structural degradation, high number of loading cycles, etc.
The question is, do any 3D printed materials meet these engineering/material properties.
Any help would be appreciated.

If this is asking particularly about 3D printing a core, not other things, please emphasize that to guide answerers.  (A Nomex-like honeycomb might be light and stiff enough, but I'm not sure about other properties.)

"I understand that 3D printing of wings, etc. is pretty common in the RC Aircraft world". Can you cite even one example?

@EricS There is special foaming filament available that is used for printing complete model aircraft.

@0scar That’s new to me and very interesting. Thanks.

There are plenty of 3D printing methods available now with FDM being the most common. In my experience as small fixed wing UAV designer, all the 3D printing processes that are financially viable always under perform in comparison to alternatives. I can tell you the main issues I have encountered leaving the printing defects out:

additive manufacturing doesn't lend itself well to mass production.
It is too heavy for a core material.
melts under heat, Fire hazard and safety issues.
FDM prints are susceptible to fatigue especially when stress is transverse to the layers
for commercial use , the end customer doesn't like 3d printed parts on his aircraft. simple as that
machined metal parts are always stronger and cheaper than metal 3D printed parts.

However, it has some major benefits but mostly as a prototyping method. You can test the fits and physical size rather cheaply. I use it sometimes as a mold pattern for small parts. Also, 3D printing wings is not as common as you would think in RC world.
TLDR; with current technology, 3D printing is not suitable for stress critical components.

"does lend itself well to mass production well." did you mean doesn't?

@HiddenWindshield edited

"machined metal parts are always stronger and cheaper than metal 3d printed parts." Always is too strong a word. For very complex shapes such as some turbine blades 3D printing is both cheaper and better than machined. Also you can 3d print shapes that are unable to be machined.

@EricS Metal 3d printing a turbine blade would be a poor design decision. Special casting methods with complex cores and gating are normally used to achieve the desired temperature resistance and strength. For compressor blades, forging is generally used.
3d printing metals is fairly expensive and will mostly be used by commercial aviation for non structural parts like a pump casing, because aircrafts maybe far from maintenance airport and time is more critical than money. Nevertheless, always is indeed a strong word

... and after printing, you'd have to machine it anyway, because you need a certain surface finish quality...

@Mridul And yet it is commonly done. https://www.ge.com/news/reports/future-manufacturing-take-look-inside-factory-3d-printing-jet-engine-parts

@EricS Interesting, we might already be in the future

@Eric S : Consider that 3D printing of titanium medical implants has become fairly commonplace.  They generally have to be strong, and have complex shapes that can vary between individuals.

@jamesqf You don’t have to convince me, it’s Mridul that is a skeptic. Relativity Space is 3D printing rocket engines.

I answered keeping in mind the context in which the question was asked, I don't know the metallurgy of inter metallic compounds GE is using nor do I know the trade offs they made, but if GE is using it must have some merit. @EricS please don't consider me a threat to 3d printing industry, I mean no harm :)

@Mridul No problem. I was a 3D printing skeptic and still am for a lot of things. However aerospace is almost the perfect fit with relatively low volumes of very high value and complexity parts. Doesn't mean anyone is going to 3D print wings anytime soon.

@Eric S: I'm not trying to convince YOU, specifically.  I'm adding some general information.  It's just that the comment format here makes it easier to keep track of things if they're responses to someone's previous comment.

I've seen several aircraft (at least the FAA would deem them to be such, even if "unmanned") that are entirely 3-d printed, so it seems that the answer to your question "Can control surfaces for slow aircraft be 3D printed?" must be "yes".
Related:
(Q) Why don't they build airplanes from 3D printer plastic?
(A) Why are geodesic airframes no longer produced?

In these days, there is a range of printing materials available, so likely most of parts that are not too big could be 3D printed. Likely not from PLA that is for parts that do not require strength or heat resistance. But say glass filled nylon or polycarbonate are suitable for hard working mechanical parts, and ultem is heat resistant and flame retardant. It is also possible to 3D print from steel or even titanium, but for any larger part this would be very expensive. You can look say here for overview of the available methods. Sizes are currently the biggest problem, the most that majority of services could 3d print are only large enough for a drone.


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What is the best prop location for distributed propulsion - pusher, tractor or blown flap for an ultralight?
What is the best prop location for distributed propulsion - pusher, tractor or mid top wing ( blown flap) for an ultralight, max speed about 60mph, for MINIMUM takeoff distance ( 20-40') and about 15mph stall speed?
I would assume Blown flap, as the wing and prop would see clean air, and the flap sees the highest air speed increase due to exit speed of the prop, assuming 100% coverage.
My assumptions are:

Tractor configuration: entire wings sees prop exit speed but air is turbulent, so less lift, prop is in clean air, so max thrust.
Blown flap (prop on top of wing): wing is in clean air, so max lift, flap sees highest increase in air speed in this configuration, so max lift, prop is in clean air, so max thrust.
Pusher configuration: wing and flap in clean air, so good lift, prop in dirty air, so less thrust

Is this correct?

If your looking for max performance in short takeoff distance, the best solution is to blow the wing with your engines. Either you chose propeller or mini distributed electrical jet fans will only change the position you will set them and both idea might actually work.
In tractor configuration the prop exit speed is actually doing most of the work to create lift. You can watch some good stall competitions videos where the plane may takes of with brakes locked. For example this video from Steve Henry

That's definitely the configuration giving you the most lift at low speed. Some other well known solution on larger airplanes including NASA and USAF experiments have the engine closer to the wing:

The Breguet BR941 had a full blown wing with 4 huge 4m diameter propellers and with flap going down to 95° allowing for less than 200m take off and 120m landing.

The NASA QSRA (Quiet Stall Research Aircraft) has again 4 engine blowing the full wing at landing to allow for more lift and allowed it to land on the USS Kitty Hawk.

Both YC-14 and YC-15 where project to reduce landing distance in the USAF that finally led to the development of the C-17 globemaster.

You are not wrong to go with tractor config.
In my opinion the best location of the propeller is pusher. The reason is that there is no airframe behind to interact with a heavily turbulent propeller wash. The thrust is developed on propeller blades alone and the airflow on the wing is the result of wing being thrusted forward in stationary air. It has nothing to do with propeller wash. The wing benefits from the most undisturbed air possible but the propeller gets the wash off the wing and airframe. For ultralight it may not be a big deal because of good thrust to weight ratio.
The tractor solution puts the engine ahead of the wing and is more common. The propeller gets the soft air and the wing gets the propeller wash. Maximum thrust, but not the best lift or drag.
Distributed propulsion means you have more contributors to propulsion.
15mph stall is similar to paramotor. You may go even lower if you’ll consider a gyro plane.

Why not the blown flap (prop on top of mid wing)? Prop and wing in clean air and the flap sees the highest increase in air speed.

I’m not used to that solution. Usually float planes worry for salt spray and place the propeller up above, for no other apparent reason.


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Do USAF fighter jets go supersonic over land?
Does the US Air Force operate supersonic fighter jets over land?
I heard that the FAA banned supersonic flight over America due to the shockwaves creating loud booms and breaking windows on the ground, therefore the Concorde only operated over the Atlantic Ocean.
However, is the US Air Force allowed to operate supersonically over land? How about in training missions, or drills, or a real event? Did they go supersonic during 9/11?

You seem to be asking specifically about america but there was a recent incident in the UK in which jets went supersonic: http://www.bbc.co.uk/news/uk-41495677 . In particular the relevant quote is "A sonic boom could be heard in Suffolk after the Typhoon aircraft were authorised to travel at supersonic speed for the operation, the RAF said."

@Chris It's fairly common for RAF interceptors to travel supersonically when responding to an incident 1, 2, 3; it also sometimes happens during training (often by the UK-based USAF, it seems) 4, 5, 6.

https://www.seattletimes.com/seattle-news/fighter-jets-scramble-sonic-booms-rattle-puget-sound-area/

I don't have any substantiation for this but I am convinced that the FAA banned supersonic flight over America in order to limit the competitive effect of the Concorde on American aircraft manufacturers and American subsidized airlines. Nobody worried about loud booms and "broken windows" before 1976.

@DavidRicherby: Yeah, I've seen other stories. I just figured the one from two weeks ago might be interesting because it is topical as well as answering the slightly different question of supersonic flights in non-us countries. Also its too late to fix it now, I know, but your first two links are duplicates.

Generally speaking, FAA does not allow supersonic flights over land except in special restricted military flight corridors. These are called High Altitude Supersonic Corridor (HASSC) in the US. For example, the one near Edwards AFB is:

NASA also mentioned about supersonic flights:

The F-18s flew in Edwards' High Altitude Supersonic Corridor at 32,000
  to 40,000 feet for the supersonic runs.

However, I read that FAA's jurisdiction is only until 60,000 feet, hence above this altitude, there are no speed limits:

According to the FAA regulations the controlled airspace extends up to
  60,000 feet. Anything flying above may fly at “unlimited speeds.”

During 9/11 attacks, US Air Force had its aircraft fly supersonically over mainland.

Presumably, they are allowed to go supersonic if there is an actual threat (not during training)

Thee are many other areas at low altitude as well, (although all are in Restricted airspace), designed for tactical training. as I recall, R-4807/4808/4809 northwest of Nellis AFB (Las Vegas), are supersonic training areas from surface to unlimited. these areas were (and still are, I would presume), heavily used by "Red Flag" Exercise operations. … And these areas are specifically designed for training.

They do, but only in certain restricted areas being used for training exercises.  The ranges north of Nellis and east of Fallon NAS are commonly used for this.  Supersonic flight over water in the warming areas offshore are common as well.
And, of course, during a national emergency.
It used to be much more common in the past.  When I was a boy growing up in Idaho, I can remember our house being frequently rattled by loud sonic booms from SR-71s leaving Beale and headed into the northern United States and Canada on training missions.  These days with the fewer aircraft in our arsenal designed for sustained supersonic flight plus a desire to be good neighbors to the local populace, supersonic flight has been curtailed quite a bit.

I remember the SR-71s leaving and arriving at Rome. People lost windows and a neighbor lost a hutch and dishes.  But we weren't supposed to know about it.  So we went to a park which was high enough to watch the arrivals and departures.  Not all flights had booms, but my guess is that the booms happened when they got low on fuel and needed to arrive faster. Also my co-workers who flew out of Beale in SR-71s said that they did go supersonic over CONUS.

Sled drivers used to fly the jet up from Beale to Nova Scotia at Mach 3, then tank, thence accelerate back up to Mach 3 and blast down the Atlantic seaboard to Miami, turning west over the Gulf of Mexico into Texas, New Mexico, Arizona, Nevada then finally back into California and landing at Beale AFB.  Total flight time: Approx 3 hours.  Total distance covered: Approx 5000 NM!

Where in ID were you? I grew up in Pocatello and don't ever remember anything resembling sonic booms.

I grew up 50 miles north of there in Idaho Falls.  This was back in the 1980s, mind you, and it often happened early in the morning there.

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text:
How safe are sail planes?
I think sailplanes have to be pretty safe. Because if it falls it will gain speed, which will be converted to lift.
So unless you do something stupid sailplanes can't just fall out of air. Am I right?

Close with lots of relevant info also some good info here

The same is true of any powered airplane.

Welcome to aviation.SE! I'm not sure what answer you're looking for: how safe is flying a glider compared to what? To flying a powered aircraft? Driving a car? Staying at home all day? Obviously some activities are inherently riskier than others, but risk and safety are usually relative.

I've never heard of an engine failure in a non-motorized sailplane, so that's one less thing that can go wrong! :)

No they can fall out of the sky if you mishandle them like any other airplane.  They just happen to be missing an engine.
What makes them safe is low landing speeds and the ability for an experienced pilot to place it on the ground with extreme precision, with a huge choice of descent angles.  If you are 1000 feet in the air, you have the option of landing almost under your nose using your dive brakes, or gliding 6-8 miles ahead, or anywhere in between.
As long as you have land-able terrain within gliding distance, and avoid stalling and spinning close to the ground (like any other aircraft), a glider is a low-stress, low risk type of aircraft. Serious cross country pilots "land out" in farm fields or outlying grass airports from time to time when they get caught low and away from the home field.  It's part of the sport.


text:
Do all flights with the exception of military show up on FlightAware?
I ask because I just saw a B737 land at KCSG. I haven't seen one land here in a while and in my curiosity wanted to know who it was. I got on FlightAware and I don't see any B737 arrivals within the last week. Has anyone got a clue as to why this is? The B737 was all white and I didn't see any obvious ID marks on it.

Related

Did you see Janet?

No.
From the Flightaware FAQ:

Can FlightAware track VFR flights?
In the US and Europe, this is not officially supported although some VFR aircraft with flight following are available on the position maps but it is largely unreliable and arrival/departure/flight plan data is often not available.

and

Can I track NASA's Space Shuttle or other space ships (UFOs, etc) on FlightAware during launch or descent?
No, but that would be rather amusing; the shuttle or UFOs would have to be on an IFR flight plan to be tracked or have an ADS-B OUT transponder. However, NASA streams live NASA TV on the Internet.

and

Can I track military aircraft? What about presidential movements in Air Force One?
FlightAware does not track military aircraft and presidential movement flights (e.g., Air Force One, Marine One, etc.) are operated by the US military.

etc.
Also compare this screenshot from FlightAware with a screenshot from another free tracking service:

You will notice that several flights are missing on the FlightAware map. The other tracking site, however, is not perfect either; these kind of internet tracking sites mostly rely on ADS-B, and do not receive radar data. Even if they did, we also still have flights which do not even show up on radar. Aircraft surveillance is complicated ...

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text:
Can hours accumulated in a glider count towards the 1500 hours for an ATP certificate?
If you have ratings in Glider, ASEL, AMEL do all your PIC hours in all categories count towards an ATP certificate, or must the Glider hours be kept separate?

According to my reading of 14 CFR 61.159, glider hours do count toward the 1,500 hour total time requirement, but not several of the other requirements. From 14 CFR 61.159 (parts in bold are where I've highlighted those requirements that must be met in an airplane category, i.e. not a glider).
Presumably you're also not logging much instrument flight time in gliders, so I didn't highlight that.

§ 61.159 Aeronautical experience: Airplane category rating.
(a) Except as provided in paragraphs (b), (c), and (d) of this section, a person who is applying for an airline transport pilot certificate with an airplane category and class rating must have at least 1,500 hours of total time as a pilot that includes at least:

(1) 500 hours of cross-country flight time.
(2) 100 hours of night flight time.
(3) 50 hours of flight time in the class of airplane for the rating sought. A maximum of 25 hours of training in a full flight simulator representing the class of airplane for the rating sought may be credited toward the flight time requirement of this paragraph if the training was accomplished as part of an approved training course in parts 121, 135, 141, or 142 of this chapter. A flight training device or aviation training device may not be used to satisfy this requirement.
(4) 75 hours of instrument flight time, in actual or simulated instrument conditions, subject to the following:

(i) Except as provided in paragraph (a)(4)(ii) of this section, an applicant may not receive credit for more than a total of 25 hours of simulated instrument time in a flight simulator or flight training device.
(ii) A maximum of 50 hours of training in a flight simulator or flight training device may be credited toward the instrument flight time requirements of paragraph (a)(4) of this section if the training was accomplished in a course conducted by a training center certificated under part 142 of this chapter.
(iii) Training in a flight simulator or flight training device must be accomplished in a flight simulator or flight training device, representing an airplane.

(5) 250 hours of flight time in an airplane as a pilot in command, or as second in command performing the duties of pilot in command while under the supervision of a pilot in command, or any combination thereof, which includes at least -

(i) 100 hours of cross-country flight time; and
(ii) 25 hours of night flight time.

(6) Not more than 100 hours of the total aeronautical experience requirements of paragraph (a) of this section or § 61.160 may be obtained in a full flight simulator or flight training device provided the device represents an airplane and the aeronautical experience was accomplished as part of an approved training course in parts 121, 135, 141, or 142 of this chapter.

(b) A person who has performed at least 20 night takeoffs and landings to a full stop may substitute each additional night takeoff and landing to a full stop for 1 hour of night flight time to satisfy the requirements of paragraph (a)(2) of this section; however, not more than 25 hours of night flight time may be credited in this manner.
(c) A commercial pilot may credit the following second-in-command flight time or flight-engineer flight time toward the 1,500 hours of total time as a pilot required by paragraph (a) of this section:

(1) Second-in-command time, provided the time is acquired in an airplane -

(i) Required to have more than one pilot flight crewmember by the airplane's flight manual, type certificate, or the regulations under which the flight is being conducted;
(ii) Engaged in operations under subpart K of part 91, part 121, or part 135 of this chapter for which a second in command is required; or
(iii) That is required by the operating rules of this chapter to have more than one pilot flight crewmember.

(2) Flight-engineer time, provided the time -

(i) Is acquired in an airplane required to have a flight engineer by the airplane's flight manual or type certificate;
(ii) Is acquired while engaged in operations under part 121 of this chapter for which a flight engineer is required;
(iii) Is acquired while the person is participating in a pilot training program approved under part 121 of this chapter; and
(iv) Does not exceed more than 1 hour for each 3 hours of flight engineer flight time for a total credited time of no more than 500 hours.

(3) Flight-engineer time, provided the flight time -

(i) Is acquired as a U.S. Armed Forces' flight engineer crewmember in an airplane that requires a flight engineer crewmember by the flight manual;
(ii) Is acquired while the person is participating in a flight engineer crewmember training program for the U.S. Armed Forces; and
(iii) Does not exceed 1 hour for each 3 hours of flight engineer flight time for a total credited time of no more than 500 hours.

(d) An applicant is issued an airline transport pilot certificate with the limitation, “Holder does not meet the pilot in command aeronautical experience requirements of ICAO,” as prescribed under Article 39 of the Convention on International Civil Aviation, if the applicant does not meet the ICAO requirements contained in Annex 1 “Personnel Licensing” to the Convention on International Civil Aviation, but otherwise meets the aeronautical experience requirements of this section.
(e) An applicant is entitled to an airline transport pilot certificate without the ICAO limitation specified under paragraph (d) of this section when the applicant presents satisfactory evidence of having met the ICAO requirements under paragraph (d) of this section and otherwise meets the aeronautical experience requirements of this section.

This was a great answer!  Thanks!


text:
Can planes fly without wings but with a horizontal stabiliser?
There are X-Wings in Star Wars and I have been making a model recently. So can planes fly without wings but with a horizontal stabiliser?

The X-Wings have wings but not a horizontal and vertical stabiliser pair. There is a reason they are called "X Wings". And on other things, the horizontal stabilisers (not elevators) will not be able to generate the force needed to turn the plane. You can have a flying wing airplane, but no flying tail. The wing being near or at the aft end of an airplane does not make it have no wings.

Boeing's Bird of Prey flew without either of those surfaces. (I don't know if there's a formal definition of a wing, but at least to me, that aircraft looks wingless -- it's all chine.)

@yshavit I think you're on to something.  I would think you could elaborate and leave a very valid answer here.  It isn't about having a "wing".  I'ts just about producing lift and remaining stable.

I don't know if X-Wings operate on planets with an atmosphere. Remember that "to fly" means using air to create aerodynamic lift or buoyancy. In vacuum, vehicles cannot fly, they are either propelled (e.g. solar sail) or unpropelled but traveling along elliptical orbits (e.g. free fall, Voyager, or ISS trajectory).

@mins Without giving away spoilers, there have definitely been scenes showing X-Wings operating in a breathable atmosphere on an earth-like planet.

Let me add two question on the X-Wing (to emphasize the nonsense design): Why do you need winds in space, and why do they fly loopings in space? Finally an X-Wing does not have to fly, because it's all simulation (VFX).

This is a question about terminology more than aerodynamics. All horizontal surfaces can contribute lift, and also the fuselage does. Take missiles, for example. They create dynamic lift like an aircraft, but only with their fuselage and fins by inclining both against the oncoming flow. Speed compensates for the small area and poor lift efficiency of slender bodies. Since a fuselage is unstable in pitch and yaw, you need to either add fins or something else which creates perpendicular forces at the far end of the fuselage to control it. Like missiles do.

AIM-9L (picture source)
An X-wing will make a horrible model aircraft. But with enough thrust even this design will fly.

A missile is an especially good example, since it pretty much is an X-wing, albeit with two X-wing pairs. I wonder if you could improve the X-wing's in-atmosphere performance by adding chines or canards.

I would think that with enough thrust, anything will fly. The real question is what is the quality of said flight? :)

@FreeMan: Would uncontrolled, ballistic flight still count?

That does, in fact, look "flyeeish". Frankly, I was getting quite concerned as the camera zoomed in toward the end - I thought that was a crew capsule. I feel much better after reading the video description and finding that it was just an engine.

I do not know about other airplanes, but, I can say that you can remove the wings on a Yack RC aerobatic plane and make a take off, climb to altitude, level off and perform level flight, perform barrel roles. inside loops, Cuban 8s, and perform high rate roles. Last you can land the plane safely. All this demonstrated on Real flight Simulator.
There are some techniques not normally used in winged flight that must be understood and used to make this wingless flight possible.
(1) The props pulling vector must be high enough above level to the ground so that the lift component of the vector is larger quantity than the weight of the fuselage. Remember, the plane is vary light as there are no wings.  Fuselages could be built much lighter as there is no wing stress on the fuselage, and the maneuvers, excluding high rate roles, are slow rate of turn and pitch attitude meaning g forces are much smaller and insignificant. Pulling out of a loop at the bottom is made possible by the tail pushing the back of the fuselage downward which raises the nose upward and causes the tail to fly below the axis of flight. Some of the lift for the pull up is distributed along the hull of the aircraft as wind hitting the bottom of the aircraft creates lift. This lift distribution along the entire fuselage distributes pull up g forces along the entire fuselage. Net vary low g force concentration. The center of lift of a wing is outboard of the fuselage, the outboard location causing a bending moment on the wing at the attachment to the fuselage.  This creates a wing breaking stress under high g forces. Also, the stress is at the point of attachment of the wing and this stress is countered by the weight of the fuselage which is distributed in front of and behind. The fuselage thus must resist the front to back centralized stress to resist breaking up at the wing junction. No such problem with the wingless aircraft.
(2) With no wings to add weight, the wingless plane can take off in vary short space.  The taller the landing gear (assuming a tail dragger design) the higher the angle of incidence of the fuselage and axis of propeller pull.  Building taller and taller main landing gear allows the plane to take off in shorter and shorter distances to the point where if the axis of the fuselage was straight up, the plane would take off straight up. With no wing weight, the plane would climb vary quickly.
(3) Until the plane's wheels leave the ground, the prop's wash and its axis of pull is in line with the fuselage, the wash hitting the elevator and rudder, the rudders upper part above the props axis. Right rudder turns the plane on the ground to the right but being above the fuselage axis, right rudder also induces left role. This role is against the turn and causes to be vary unstable, the plane susceptible to simply rolling over on its side if the width of the landing gear is not enough to counter the role vector. It is suggested that a wingless plane have wide landing gear for this reason.
(4) Once the wingless plane leaves the ground, the nose initially rises while the tail is still on the ground.  This makes a steeper angle of the fuselage to the planes direction and the prop wash (as seen when a smoke screen is used on the front of the fuselage) completely misses the tail sections, the wash flying over them.  The axis of role of the plane is concentric with the prop wash. Now, when the rudder is turned to the right, the vector of the rudder turns the plane to the right on the direction of flight axis.  However when turning right, the rudder being below the axis of role, induces a role moment on the axis of flight that roles the plane to the right. In this nose up fuselage angled below the axis of flight, the rudder performs the turning function and the role function. If the flight speed is too fast, the axis of flight and the fuselage axis close together and the rudder behave in turn in a direction opposite the rudder input. This flying with the nose to low causes loss of control and can result in extremely fast roll rates. It thus becomes  paramount that the wingless plane be flown with vary high nose up attitude. In level flight the speed building up will start leveling the fuselage and result in loss of control and stability. To counter this leveling off as speed builds up, simply lower the power setting.  This also means that the wingless plane will not straight up as the fuselage angle to flight angle will be 0 and the rudder will cause loss of control.  At the top of an inside loop, the tail will pull the tail over the top maintaining the rudder's role and turn function.
(5) Turning:  The rudder in nose high attitude will swing the nose but it will also bank the plane. Initiating right rudder  will bank the wingless plane to the right. At this time, the operator simply pulls back on the elevator which causes the plane to turn similar to a winged plane, a coordinated efficient turn.
(6) Landing: A few prayers help! To loose altitude don't push the nose down with the elevator as this will cause the tail to rise up onto the prop wash creating instability and loss of control. Rather, pull back on the power maintaining full up elevator. the plane will sink at your desired speed. The plane will appear to hit hard on touchdown, but the front wheels and the tail wheel hit together or tail wheel first absorbing impact. Also, remember that the weight of the wings is absent. The plane can land at a vary high sink rate without destruction of landing gear or broken fuselage.  I have broken many main landing gears landing planes hard with wings. To date, I have not broken one landing gear landing many times wingless. Remember, when landing with no wings that the moment the tail and mains are on the ground, the prop wash and axis of flight are level with the rudder and the plane is prone to tipping over when rudder is actuated. Use rudder judiciously until the rolling speed has slowed down.
(7)I suggest that all aerobatic airplane have split control on the elevator. The elevator is in the prop wash and is effective in slow speed nose high flight. When right aereolon is initiated, the elevator's right side will turn down slightly and the left side will turn up.  This splitting could be done by putting separate servo controls on each independent side of the elevator or 2 the push pole control servos could be run to each side of the elevator but the servos could be on attached to each other and a third servo could pull the two servos to actuate elevator function. This would give back to the airplane's operator the ability to control bank, the bank not dependent on being below the airflow of the prop and axis of flight direction of the flight.
Can the plane perform in the real world with the above modifications?  I believe so. Wingless flight with split elevator would open a complete new world to freestyle Rc flight. Wings could be made to be easily removable before takeoff or could be made  to be dropped off during flight. Pilots could develop no wing flying skills using the  Real Flight Simulators, the skills developed  while going through the inevitable crashes without having to rebuild expensive equipment.  Just hit the reset button!
Bottom line, the answer is a surprising YES!
Below is 17 minuet YouTube video I published with aerobatics and without wings:

Spike Selig Long time pilot and flight dynamics specialist

An aircraft needs sufficient force to overcome the force of gravity.  This doesn't have to come from a wing.
A good example are helicopters, that generate sufficient force by using a propeller to push a column of air downwards.
The RAF Harrier Jump jet, which the British used in the 1982 Falklands war does have wings, but is able to continue flying in situations where the wings do not generate lift, by redirecting the air from its jet engines downwards.
There are classes of aircraft that generate sufficient lifting force by being lighter than the mass of the air they displace. Hot air balloons and airships fit in this category.
However, the most interesting category is where the shape of the aircraft body actually generates lift. Current examples are the high speed gliders being designed by companies designing orbital landing aircraft, where having a wing is problematic for re-entry into the earths atmosphere. These are mostly blended wing designs where the lift from the body allows the design to have a smaller wing.
lifting bodies

Helicopters have wings. They just rotate very fast :p

The difference is, helicopter blades do not provide any lift by low air pressure pulling the helicopter up, it is all pushing air downwards. Verified by flying a helicopter just above a giant pair of scales. The scales read the same weight after the helicopter lands.

authors: Давид Пономаренко


text:
What is a IATA aircraft sub-type?
I am currently reading docs regarding slots, where the following is mentioned:

All SCR messages in linked (turn-around format) using aircraft
subtypes only (e.g. 73G or 738 instead of 737).

Not sure what is meant by the above sentence. My understanding is that whenever you submit slot requests with SCR messages you need to use the IATA aircraft code as defined here.
The Worldwide slot guidelines also mentions:

Operating a flight in a significantly different way to the allocated
slot – including a different service type, aircraft subtype, aircraft
capacity, or origin/destination – without the prior confirmation of
the coordinator as set out in 8.10;

So what is exactly the difference between an aircraft type and an aircraft subtype? Does the link provided above refer to aircraft subtypes?

When the 737 is a type, the 737-700 (IATA: 73G) and 737-800 (IATA: 738) are sub-types. They have different capacities and differing weights. Other documents will use the word "variants" where your document uses "subtypes".
Related info here.

Ralph J thanks for the clarifications!


text:
Are shrouds and stators needed to maximize thrust from stacked props and motors in electric aircraft like the Lilium Jet?
Do some e-vtol aircraft, like the Lilium need shrouds and stators to maximize thrust from stacked electric motors and props?

The aircraft is actually called the Lilium Jet, although it does not heat the air passing through the props by burning a fuel like in traditional jet engines. Not sure if that really qualifies it to be called a jet.
I understand that the Lilium jet, NASA's X57 and the Opener Blackfly use a concept called distributed propulsion, to accelerate air over a wing.
Eg. If I want to fly the Blackfly at 28mph with a single prop, according to my calculations, I would need a wing with about 180 sq ft ( the Lazair had 140 sq ft, or a wing loading of about 3 lbs/sq ft).  Using distributed propulsion, the wing is seeing about 2.5 times that speed, so it only needs 40 sq ft or about 10 lbs/sq ft of wing loading.  The smaller the wing area, the less drag at cruise speeds, so the smaller the wing area the better.
What if I stack 2 motors and props axially?
The exit velocity of the first prop (about 70 mph, or about 230% increase), becomes the inlet velocity of the next prop, which in turn increases the exit velocity (say only 130% increase, the higher the inlet velocity, the lower the increase), say 90mph.  Now I can size my wings for a minimum velocity of 90mph, or maybe 20lbs/sq wing loading, even though my aircraft is still only flying at 28mph.
Does this work?
Would it work or work exponentially better with a shroud?
What if I put a stator ( eg, a "fixed 8 bladed prop" with blades arranged opposite to the rotation of the previous prop) in-between the props inside a shroud to slow down the velocity of the inlet velocity of the next prop and increase the pressure.  Now the exit velocity increase of the 2nd prop would be much higher (also around 200%??).  Now I can add an expansion nozzle at the end of the shroud, to convert that increase in pressure to an increase in velocity. Now I could resize my wings for maybe 200mph minimum speed, or about 60-70lbs/sq ft wing loading, even though my aircraft is still only flying at 28mph.
Essentially you're building a jet engine, but without any heating.
Does this work, at least conceptually, even though you're not heating any air with fuel?
NASA's X-57 only has a single row of folding props, so I imagine that this is why the shrouds are not needed.

We have a number of questions already about the use of shrouded/ducted fans, e.g. here, here, here (and others linked from them). Do any of them help?

Not really. I've adjusted the question slightly. I'm really asking if stacked props need shrouds.

The Lilium Jet uses ducted fans, but turbofan engines are essentially the same with a different power source.

The main problem with a setup like this is it can’t really glide. You would never be able to land conventionally as you would have to increase thrust to get the lift needed to land. This isn’t a problem when taking off but when landing you need to be slowing down not speeding up.
Also the benefit of smaller wings would be quickly outweighed by the very low propulsive efficiency of the fans. Compressing air and expelling it via a convergent-divergent nozzle is very inefficient for sub sonic flight. It would likely take 3-4x more power for the same thrust.
To answer your question in the comment there are many examples of counter-rotating props without shrouds.


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How are foam cores shaped for tapered and swept back wings?
How are foam cores shaped for tapered and swept back wings?
I understand that many constant chord, constant thickness rectangular wings use the same sized airfoil shape from root to tip. Let's assume the wing uses a classic NACA airfoil shape such as 650-18.
How would a foam core wing with a classic NACA airfoil shape, swept back 10 degrees be shaped?  Is the wing, using the same airfoil shape, NACA650-18 just rotated back 10 degrees from the root?  In this case, in cross section longitudinally (along the plane's y axis, in the direction of airflow), the actual airfoil shape would no longer be the same shape as the rectangular non swept wing, even though the geometric perpendicular cross section of both wings is exactly the same. Is this correct thinking?

It isn't clear to me whether you are asking about manufacturing techniques, or wondering about the altered shape definition resulting from the longer chord when the wing is swept.  You are correct in your thinking that the relative or apparent chord of a straight wing becomes elongated when swept.  Is that all you are wanting to confirm?

I was asking about both actually, the manufacturing technique, and how the shape is altered.

I'll assume you're already researched the methods for hot wire cutting foam blocks in the Rutan moldless method and are familiar the hot wire tool and airfoil templates you make and the template cutting process.
You can do it two ways; make hotwire templates with the longitudinal axis airfoil contour (the stretched airfoil shape resulting from the sweep) to stick on the end of the foam blocks that you have already truncated with the 10 degree sweep, or make the templates to the airfoil profile perpendicular to the mean chord line, stick them on to the blocks with square ends, and truncate them to the 10 degrees sweep angle after you've cut them out.

So how do you account for sweep by elongating the airfoil template as in your case 1 above? I assume you just use the same airfoil shape, but with a longer chord? In either case, the "longitudinal airfoil shape" will not be the original one. Is this correct?

Take a straight wing and cut through it at a 10 degree angle. The outline along the 10 degree plane is elongated because of the truncated cut.  That is the airfoil profile to the airplane, because that is the profile that is aligned with the airflow.  That's all sweep does; it makes the air think the airfoil is longer for the same thickness. I'm not sure if the Rutan procedure and the templates that came with the plans were for method 1, template parallel to the airflow profile, or method 2, template perpendicular to the spar profile. It should be easy to find a builder's web page showing it.

So if you're designing an airplane from scratch, do you use the elongated chord in your calculations? Also, seems to me, you can't use the standard lift curves because the shape is different - different lift, drag and moment curves. Is this correct?

I assume you mean the chord is longer, the span would actually be shorter.

@Fred, I would suggest that you incorporate your question about calculations into the original question.  Because that was why I was uncertain what you were looking for...


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Is it recommended to forward slip an aircraft wth a v-tail?
Is it recommended to forward slip an aircraft wth a v-tail just above stall speed at a maximum angle of attack?
A forward slip requires crossed controls, eg. Full right rudder and full left aileron and then adjusted to maintain forward flight.
However executing at maximum angle of attack requires "full elevator", which in the case a vtail, is also the rudder acting as the elevator.
As a side note, forward slips are not recommended by the manufacturer of Velocity Aircraft, an aircraft with twin rudders, as the leeward rudder may become in-effective as it may blanketed by the fuselage and become stalled or have unclean airflow.
So is this recommended? I would think no, as it would have the same problems as the Velocity.

You don't forward slip at all close to stall. You will be at your final approach speed, roughly 1.3 Vₛ, and when you slip you will normally add a few knots of extra energy as a safety margin by lowering the nose at the same time.
The only time you are in that condition close to stall is in the landing flare in a crosswind, side slipping to counteract the drift. The V tail airplane has to be as capable of doing that as any other. The main effect that owners of V tails have to live with that is unique to the configuration is annoying lateral yawing moments in turbulence. The V tail Bonanzas are famous for this.
As far as control authority for rudder and elevator goes, the range of motion of the control surfaces (called "ruddervators") is quite large and the ruddervator surfaces are oversized (bigger than a regular horizontal tail would be, but smaller than the horizontal tail plus vertical tail), to accommodate the need of the control linkage mixer unit to sum pitch and yaw inputs to achieve the same result as a regular tail.
Say, for example, you apply full up stick with no rudder; both ruddervators are up about, say, half-travel. Adding left rudder pedal while holding full up stick will make the right ruddervator go up more while the left ruddervator goes down a similar amount (that is, less up), to create a net nose left yaw moment without changing the net pitching moment.
Apply full right rudder with the stick at neutral, and the left ruddervator goes half up, and the right ruddervator goes half down. Pull full back stick while holding the rudder input, the left ruddervator goes up more, while the right ruddervator comes up to a less down position.

V-tail or not:
Flying at full elevator deflection, at maximum angle of attack isn't recommended unless you are intending to demonstrate stall recovery.
Flying at full elevator deflection, at maximum angle of attack and then entering unbalanced flight using rudder isn't recommended unless you are intending to demonstrate spin recovery.

This is a good answer, but I think it can be made even better if you add information specifying why a V-tail does not result in a difference.

@EndAnti-SemiticHate, thanks for the feedback, but:  1.)  I'm going for brevity and pointing out the [obvious] risk in any aircraft, and 2.)  I don't have the specific aero expertise to explain why the risk is not less with a V-tail.   I'm just going to leave it as-is.


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What does the "MAX EMR PWR" lever do in a Quest Kodiak?
On a photo of the Quadrant of a Kodiak Quest 100 there was an additional red lever to the left of the throttle (black handle) labeled with "MAX EMR PWR". Can anyone explain what what this means and what the corresponding lever is used for?

Picture by Jeff Berlin on flyingmag.com

FYI, it isn't the "MAX EMR PWR" lever, it is the "EMR PWR" lever, with two positions:  MAX, and NORMAL.

It engages the afterburners.

There is already a great answer quoting from the aircraft flight manual, but I thought I would offer some additional perspective.  I'm not qualified in the Kodiak, but both the T-34C and C-208 have versions of the venerable Pratt and Whitney PT6 turbine engine, and both have Emergency Power Levers like this in case the Fuel Control Unit (FCU) fails.
Older (non-FADEC) turbine engine FCUs are electro mechanical, and meter fuel according to several inputs; typically RPM, turbine temperature, demand, (per power lever position) and probably a pressure sensor somewhere.  With a failure of the FCU the engine will still run, but the metering function no longer works and the RPM will roll back to idle.
This is when the Emergency Power Lever comes into play:  Once engaged the EPL bypasses the FCU and gives you manual control over a valve to add fuel to the engine to increase or decrease power output.  However, without the other sensor inputs you lose the governing protection of the FCU.  Instead of the power lever being just one input to be considered by the FCU to change a power setting, you are now directly controlling the fuel flow going to the manifold.
The way it was explained to me was to imagine your car's carburetor or computer controlled fuel injection system failing:  Instead of venturis, needle valves, jets, etc. keeping a uniform fuel mist flowing into the cylinders, you are now keeping it running down the road by carefully and slowly pouring gas from a spouted measuring cup directly into the intake manifold.
Needless to say you must be very careful using EPL to dump raw fuel into the burner like this or you can easily overtorque the engine or toast the turbine section.  In Navy flight training they had us fly an approach in the simulator using the EPL and it was VERY sensitive.  You really had to keep an eye on the max turbine temperature and move the EPL very gingerly to avoid exceeding limits.

pouring fuel from a measuring cup into the intake manifold sounds like a comedy skit. Someone should do that. Bonus points if it's on a plane.

In high school a friend of mine had the throttle linkage on his old Nova break.  He tied his shoe laces together, tied it so the throttle lever on the carburetor, threaded it through the firewall, and used a pull string throttle to get home!

Oh good, a Nova is not a type of plane

Yeah sorry I didn't make it clear the Nova was a car!  I am of a certain age when they were very common...

I knew exactly what you were talking about.

I heard it said that in olden days bush pilots with a total fuel failure could rig up a pipe from the fuel tank ending in front of the air intake of an engine, such that the intake would suck in a fuel air mix like a carburetor. No control but it might get you home


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Can a safety pilot acting as PIC also log cross country time?
I am working on my instrument rating and I need to build up both my instrument time and Cross Country time to meet the requirements.  I am planning on making most of that time by flying with a friend who I will switch off between hood time and safety pilot.  We are planning on doing lots of cross country hood time to meet our hours.  Also the safety pilot will be designated PIC so that we can claim PIC time as safety pilot.
We need to build 50 hours PIC cross country time so my question is can the safety pilot, if they are designated PIC for the flight, claim cross-country time even if they are not touching the controls for that flight?

"Also the safety pilot will be designated PIC so that we can claim PIC time as safety pilot." - You do realize that the legal pilot in command will be responsible for the other guys flying if there is a violation or accident.  The logbooks will tell the story to the FAA on who was the legal pilot in command.

The safety pilot cannot log cross-country time.
The FAA has issued a Legal Interpretation (Gebhart) that clarifies the FAR.

Section 61.65(d) contemplates that only the pilot conducting the
  entire flight, including takeoff, landing, and en route flight, as a
  required flight crewmember may log cross-country flight time. Because
  a safety pilot does not conduct the entire flight, a person acting as
  a safety pilot for a portion of the flight may not log any
  cross-country flight time for the flight.

According to FAR 61.1, one of the requirements of cross-country flight is that it is flight time that includes a landing at a point other than the point of departure. Presumably, no one is under the hood at that point, and so only the pilot who makes that landing is logging time at that moment, and so only they can log it as cross country time.
XC time also must include the use of dead reckoning, pilotage, electronic navigation aids, radio aids, or other navigation systems to navigate to the landing point. Which one of you is doing that?

I'd also add the aside that this is a really bad way to learn instruments. Presumably, you shouldn't have to "build your instrument time" by flying with a safety pilot before getting your rating. That's an excellent way to establish bad habits.

I think that training with a safety pilot who is also working on the rating makes both better instrument pilots. After you learn the basics of instrument flying the rest of the hours are just learning to maintain focus and consistently fly the approaches. Unless you are really bad at instrument flying, the CFII is just acting as an expensive safety pilot for most of the 40 hours.

For Commercial and instrument ratings 61.1 (ii) you only have to navigate to a point of landing that is 50nm away. If you do a zero/zero takeoff and then perform a enroute procedures under the hood shoot the approach down to MDA. That in and of itself counts as a cross country for the pilot manipulating the controls, without landing. So if the safety pilot was the take over and conduct the landing, that would be technically and cross country for both pilots? Seeing as the safety pilot was required for the takeoff, enroute procedures and landing.
"Includes a landing" and "includes a point of landing" are two different things.
(ii) For the purpose of meeting the aeronautical experience requirements (except for a rotorcraft category rating), for a private pilot certificate (except for a powered parachute category rating), a commercial pilot certificate, or an instrument rating, or for the purpose of exercising recreational pilot privileges (except in a rotorcraft) under § 61.101 (c), time acquired during a flight -
(A) Conducted in an appropriate aircraft;
(B) That includes a point of landing that was at least a straight-line distance of more than 50 nautical miles from the original point of departure; and
(C) That involves the use of dead reckoning, pilotage, electronic navigation aids, radio aids, or other navigation systems to navigate to the landing point.

I think you would be hard pressed to find an FAA ASI or DPE who would agree with you on that interpretation about cross country time.

It’s the same reason why when students conduct there 250 nm ifr cross country they don’t have to land anywhere other than where they took off from. It is all still considered a cross country. They shoot the approach don’t land and continue with there flight plan.


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Why pull up on the rollout after landing, after the nose wheel is down?
I'm a student pilot learning on a Piper Cherokee 180.  When I land and the nose wheel touches down my instructor wants me to keep pulling back on the stick through the landing roll.
For soft field landings this makes sense as you don't want the nose wheel to dig into the ground and tumble over.  However we are landing at an airport with hard surface runways which routinely experiences gusty wind conditions.  It seems to me that if I keep pulling up and a gust of headwind hit us shortly after the nose wheel touches down that the gust would lift the nose wheel back off the ground.  This of course would not be a good thing.  So in my mind I think it would be better to ease off the back pressure once the nose wheel touches down.  What are your thoughts?
Note:  My instructor was a crop duster so I think he just carried over the soft field landing technique and is just teaching this because this is what he learned.

Keeping the flaps down and the nose up slightly lets the wings help slow the plane down. You don’t need to use the brakes as much (or at all).

see aero braking https://aviation.stackexchange.com/questions/51834/is-aerodynamic-braking-really-effective

Why not just ask your instructor?

It is a soft field technique but is generally applied on hard surfaces for the sake of the aircraft. There are a few main reasons;

Take stress of the nose wheel and nose gear assembly which is a bit more fragile than the main gear assembly. This is particularly true in aircraft equipped with a steerable nose wheel like the Cherokee (or in retracts).
Provide adequate clearance for the propellor. If you look really closely the Cherokee has a low enough clearance such that, excess low pressure on the nose wheel and a sufficiently hard landing can cause a prop strike. This is one of the reasons its so important to check tire pressure in your preflight.
It also helps to reduce the risk of the fated nose wheel bounce which can collapse the assembly.
Generally it also helps to prevent you from simply releasing the controls after the mains touch down or potentially even pushing forward on the column which would drive the nose down and potentially cause damage.

During training this also helps to build habits that lead to a good landing attitude and speed bleed off especially if you start out landing flat.

While these are valid reasons, the practical effects of holding back after the nose is down are debatable.  It's more important to put the nose down gently than to hold back pressure afterwards.  It does help to reduce nose wheel shimmy; however, if you're getting that then your plane needs maintenance anyway.

I'm not an expert, but would pulling up also mean more drag, so you brake faster? As the airplane needs to spend energy to convert it into lift, that normally would be dissipated through ground contact

As long as you aren't pitching up when pulling back it also means more down-force on the rear gear which improves braking performance.

@Undo this may be worthy of a separate question but its always hard to say what decision he made. Its clear he made the decision not to apply full power and execute a go around which he should have.

Is #1 so important? I was taught to pull the stick back after all wheels are on the ground, in a taildragger. And taildraggers have their weakest wheel in the rear.

@vsz tricycle gear aircraft have different considerations. In a tail dragger you need to worry about ground looping and nosing over the aircraft which are bigger issues than rear gear problems.

You don't even need low pressure in the nose wheel to get a prop strike on landing.  One of my partners managed to do it :-(

Aerodynamic slowing is more effective than wheel-braking at touchdown speeds, and aerodynamic directional control is more effective than pedal steering until you decelerate enough that you can't keep the nosewheel in the air.  If you're flying a differential-steering (or "brake-steering") airplane, this is even more important.   Using this technique, you would push forward to neutral elevator once the elevator loses effectiveness, so the headwind gust should not affect you much.  Even with your nosewheel on the ground, if you experience a gust, you'll still need to "fly" the airplane (remember the taxiing rules).  Any time you have all 3 wheels on the ground, you're taxiing (even if you're on the runway).  If you keep the mindset that you are "flying" all the time, then you're unlikely to lose control when you encounter a gust or wake turbulence.

And holding the elevator up helps with wheel braking as well, as it shifts the aircraft's weight from the nosewheel to the main gear (where the brakes are).

@Sean This is a myth. Apart from a prang, the nosewheel at most has ten percent of aircraft weight on it. Any action that would reduce that weight will increase wing angle of attack, creating more lift than what could be theoretically transferred to the mains. Back pressure on the stick on landing is useful for aero braking, as the answer states.

Other answers have covered the tricycle gear situation well - and of course that's what you're flying - but you mentioned that your instructor was a crop duster and it's possible that he's carrying over some tailwheel technique into your training.
In a tailwheel aircraft, it's normal to keep the stick fully back throughout the landing roll and even during taxiing to get more weight on the tailwheel and improve directional control on the ground as the airspeed bleeds off and the rudder loses control authority (see this question). Many crop dusting aircraft are tailwheels, and it may be that your instructor is doing it out of force of habit.
I'd suggest just asking him about the technique - in a neutral way! - and see what he says.

"as him about the technique" as in "This seems counter-intuitive to me (for reasons in OP) - can you explain further the reasons for it so I'm not confused."

Short answer, listen to your instructor.
Next, evaluate the landing sequence and what it would take to lift your plane off the runway after it touches down.  You will note that at approach speed
and when you flare, the AOA to stay in the air is
much higher than after your nose wheel is down.
The only time there would be a significant risk of being lifted back into the air would be when the mains were down and the nose was still up. Here your question is very valid and you may wish to work out
a procedure with your trainer to deal with this.
A bounce when you land is a similar situation,
airspeed still a little high.  Uber important to
land on the much stronger mains and keep flying the plane as it settles onto the runway.
Aside from what are you doing flying in that type of
weather, part of your plan could be to initiate a
full power - go around if you get lifted more than
a few feet.
If your procedures allow, you may also consider
immediately retracting flaps as soon as your nose
is down to further lower your AOA to a headwind
gust.
But under normal conditions holding the nose off
on your roll out will reduce stress on the nose wheel and give a little more aero braking.
Yes, if your nose rises unacceptably, it is good to relax back pressure on the stick, practice will help
one know how much.
Bob

This has been alluded to in a comment, but to put it in an actual answer: one of the primary reasons for holding the stick back during rollout on a tricycle configuration such as the Cherokee is increased brake effectiveness. Holding back the stick causes more of the weight to be born by the main gear rather than the nose gear. The main gear have brakes. The nose gear does not. Therefore, putting more of the weight on the main gear makes the brakes more effective.
Additionally, prior to the nose gear touching down, holding the nose high after the mains touch down causes much more drag, too. This is known as aerobraking and is a quite effective method of slowing down the aircraft. Aside from helping to slow the aircraft more quickly, aerobraking is also beneficial due to the fact that it dissipates the aircraft's energy into the air (which doesn't wear out) instead of into the brakes (which do wear out and are expensive to replace.)

And from a comment on that comment: This is a myth. Apart from a prang, the nosewheel at most has ten percent of aircraft weight on it. Any action that would reduce that weight will increase wing angle of attack, creating more lift than what could be theoretically transferred to the mains. Back pressure on the stick on landing is useful for aero braking, as Roberto's answer already states.

@Pilothead Increase in AoA once the nose gear is down would be relatively minimal. It would only be from less strut compression on the nose gear. And what AoA increase did exist would also add to aerobraking. More importantly, holding the stick back means the elevators/stabilators will be deflected to cause a downward force on the tail. This will add to the force on the mains and does indeed increase braking effectiveness. Granted, all of these effects will diminish rapidly once the tail stalls, but that will happen after the wings stall.

The strut has 4in of travel on a 90in wheelbase. Unloading the strut results in a 2.5deg increase in angle of attack. At any speed where the tail is still capable of raising the nose, the increased wing lift will exceed the 200lb max you can unload from the nosewheel. As I said, I agree with aerobraking.

Your instructor was likely taught, as I was that the nose wheel is as fragile as a twig and that you need to keep pressure off it at all times. In reality there isn't that much pressure on it during a normal touch down and roll out. As a new pilot and wanting to be as proficient as possible I have videos of most of my landings (Piper 140) taken from the rear tie down facing forward. Even on the odd cough cough "firm" landing with full flaps the struts barely compress. Only a small percentage of the total weight isn't still being held up by lift at touch down. Most firm landings "feel" way worse that they really are. Even during roll out when the wheels are on the ground and speed is down near 40kts there is still tons of lift. When you dump the flaps, that's when the plane settles down to almost "normal rest" position on the struts. It's quite dramatic the drop. If the winds are light I almost always keep the flap down until I'm not reading airspeed any more. I also use back pressure as a very effective brake during the roll out. The point to my long winded response it that there are multiple ways to protect the nose wheel.

Hi Chris. This is an interesting anecdote, but I'm not sure it answers the question at the top of the page. Please note that the Stack Exchange format is meant for a single question, and answers to that single question; not forum-style ongoing discussion. Answers that don't attempt to answer the question can be deleted by our experienced community member. Maybe you can [Edit] this to clarify how it addresses the original question?

When the nose is pulled up, it increases the reaction on the main wheel.
 The braking friction force is given as
Friction = (braking friction coefficient) * reaction
Therefore, by pulling the nose up, you are increasing the friction and there will be a reduction in the stopping distance.


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Will I have to redo flight time for switching to a new CFI?
The CFI that I am flying with is experiencing health issues and may not be able to fly anymore.  If I switch to a different instructor will I have to redo flight time with the new instructor or is it perfectly OK to log flight instruction with different instructors.  So far I have only about 4 hours instruction and have not soloed.  My jurisdiction is the USA.

You will not need to repeat the hours. I flew with two different instructors during my training due to my first one heading off to the airlines and it was not an issue, as a matter of fact its pretty common. The FAA considers flight time, flight time. Once you logged the hours they are yours for the rest of your life.
However the new instructor will most likely have you repeat some of the stuff you went over with the first CFI just to see where your skills are at. If you only have 4 hours under your belt is likely you have just done turns, climbs, decents, and possibly stalls so switching instructors will be no issue. It takes a fair bit of time to build a rhythm with an instructor anyway.
Just some advice, if you are at a flight school that has a few others to pick from its worth it to ask them what their intentions are. Most instructors are there to build hours towards ATP then its off to the airlines. You should ask instructors how many hours they have left if they are going to ATP out, the more hours they have the longer they will be around. This will help you prevent the situation where you have to change instructors again.

So far I have done climbing and descending turns, Power Off and Power On Stalls, straight and level flight and turns in straight and level flight.

Your flight time expires when you expire!
Beyond that, your new instructor may be the one that has to "sign you off" when your training is complete so regardless of the number of hours you have with someone else (even beyond the minimums), you must still perform well enough that that instructor will sign your 8710 for you. There are some regulations concerning how much time an instructor has to have with you before signing you off. You can find those in FAR 61.103.
FWIW - My wife went through five instructors (for various reasons) and each instructor wanted a quick flight to see where she was at in her training. Switching instructors happens for a variety of reasons. If you are not comfortable, switch. It is your money and your training!

"Your flight time expires when you do!" - What do you mean like this? It sounds like you're saying that if you switch CFIs, then your previously earned hours no longer count. That's not true, is it?

@tannerswett a person expires when they die.

@T.J.L. is correct in my meaning.

Even after the clarification, it took me a good minute to figure out what you meant by "your flight time expires when you do". I've submitted an edit request, changing "when you do" to "when you expire".